Add an OutputStream.closed getter.

tests/standalone/io/list_output_stream_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #import("dart:io");
 #import("dart:isolate");
 
 void testEmptyListOutputStream1() {
   ListOutputStream stream = new ListOutputStream();
   Expect.equals(null, stream.read());
@@ skipping 121 matching lines @@
     new Timer(0, (_) {
       result.add(3);
       stream.write([4]);
       stream.close();
     });
   });
 
   donePort.receive((x,y) => donePort.close());
 }
 
+void testListOutputStream5() {
+  ListOutputStream stream = new ListOutputStream();
+  ReceivePort donePort = new ReceivePort();
+
+  stream.onClosed = () {
+    Expect.isTrue(stream.closed);
+    var contents = stream.read();
+    Expect.equals(3, contents.length);
+    for (var i = 0; i < contents.length; i++) Expect.equals(i + 1, contents[i]);
+    donePort.toSendPort().send(null);
+  };
+
+  Expect.isFalse(stream.closed);
+  stream.write([1, 2, 3]);
+  Expect.isFalse(stream.closed);
+  stream.close();
+  Expect.isTrue(stream.closed);
+  Expect.throws(() => stream.write([4, 5, 6]));
+
+  donePort.receive((x,y) => donePort.close());
+}
+
 main() {
   testEmptyListOutputStream1();
   testEmptyListOutputStream2();
   testListOutputStream1();
   testListOutputStream2();
   testListOutputStream3();
   testListOutputStream4();
+  testListOutputStream5();
 }